The long-term objectives of this research project are to identify and characterize the mammlian form of the amphibian peptide physalaemin and to determine its physiological and pharmacological mode of action. The initial focus has been develop a scheme to purify this immunoreactive substance from kilogram quantities of tissue. Dry rabbit stomachs, which contain the highest amount of the physalaemin-like immunoreactivity (PSLI), were first extracted in an ethanolformic acid solution to remove interfering mucins. A concentrated and clarified extract was passed through cation and anion exchange resins, molecular sieving chromatography and absorbed on reverse-phase supports. The most purified preparation to date is a colorless solution or a white powder in the dry state, and a single absorbance peak can be attributed to an immunoassayable quantity. Final proof of its purity requires several more chemical analyses. PSLI differs from physalaemin, however, based on the following chemical traits: neutral, hydrophilic, not readily absorbed on glass, an apparant larger molecular weight, differentially cross-reacts with distinct physalaemin antisera, and contracts guinea-pig ileum about 20-25% of that observed with physalaemin. This hitherto unknown neuropeptide may thus prove to be a prototype of a new class of mammalian peptide hormones.